Horford is what he is. While he elevates everyone around him, he himself is elevated by the players he is with.
He is at his best when he is fully within a great system. He isn't a put the team on his back kind of player. Surrounded by great players, he can score 15-20 while putting up 5+ assists at the center position. Give us his defense, and bam he is worth or close to worth his contract.
Surround him with young or not great talent, and he isn't worth the 30 mil.